How will I know when a Krell KSA-80b needs to be recapped?

If you hear a 60 hz hum and the amp sounds soft on the low end, then its time for replacement. If not -- and the amp sounds normal with no visible damage or bulging -- the caps are fine and whether you replace them at that point is up to you. Just be aware when you send in an old amp with crowded boards for recapping, other parts tend to get replaced that are in the way, which is a factor why these repairs run in the three figures.

14 years isn't too old yet if it's been always used.

I'm pretty sure the KSA-80b is more than 14 years old. It's probably closer to 25-30 years old.

Yeah, the KSA 80 first came on the market in 1987 or 88, as that's when I bought mine, so I'd say it's in the 25 year old category.  Really nice amp; it might be ready for a recap or tuneup, but it would be worth the price, as there is not much in that price range I have heard in solid state amps that sounds better or can drive difficult loads as well.